Update from Jolie on Sin City 2

Initially she was in talks to play a lead role in Sin City 2, but she was pregnant at the time and director Robert Rodriguez was going to push back the start date, but instead started working on Grindhouse with Tarantino. It looks like the film is still "up-in-the-air" until Rodriguez finishes Grindhouse and he can start working fully on it. These are some of the specific things that she told CS.net:

"We talked about it and I read the comic," Jolie told us. "I don't think the film is being made at this moment. When it's actually going to be made I'm sure we'll talk about it."

"It was a funny thing, because the idea came to me when I was pregnant," she added. "So it was this idea… I've been Clover (in 'The Good Shepherd'), depressed and quiet, and then I was feeling very maternal, pregnant. It was this idea of this sexy, violent and loud [character]… I thought maybe after I'm pregnant it would be nice to do."

As ComingSoon.net also states, Jolie would be playing Ava Lord in the film. In Frank Miller's graphic novel, Ava Lord is an ex-lover of Dwight McCarthy, played by Clive Owen, who manipulates men through her good looks and her supposed innocence.